I have a Remington 870 super mag that is not fully locking up when you cock it unless you slam it way harder than I believe you should need to it looks like the extractor is not moving easily enough. Do I need a new extractor?

Sounds like something is obstructing the spring or plunger have you taken it out and cleaned it? I have seen this happen before from that. Does it move freely? Feel gritty?
I am sure you have checked all of these things but just incase....

Barrel not fully seated and secured with the mag tube/barrel retaining nut. More common on tight new, guns.

Forearm nut loose. Some plastic forearms are very tight on the slide tube, and a loose nut will allow them to move rearward, when cocking, and, unless slammed forward, may not fully close the action on the forward stroke.

Shell Latch out of place. I hope not- this usually means something has gotten bent or galled, if you've manually ridden over the "obstruction".

Questions:
Is it a new gun?

Have you recently disassembled it?

Has a fair number of rounds through, but never taken down/detail stripped?

Had this problem with my 870. I took it apart and deep cleaned it and snuffed the forend nut down. That fixed it for me.

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